Effects of Intermetallic Phase Formation and Bilayer Spacing on the Oxygen Exchange Process in Mg-CuO Reactive Multilayer Films

The present work examines reaction and oxygen exchange processes in magnetron-sputtered Mg-CuO nanolaminates as a function of bilayer spacing. Energy production in stoichiometric Mg-CuO films with bilayer thicknesses between 170 and 1020 nm is examined via differential scanning calorimetry (DSC), and classical Kissinger analysis methods are implemented to determine effective activation energies (E_a). To elucidate structural evolution in the Mg-CuO system, select samples are analyzed via in situ high-temperature X-ray diffraction (HTXRD). DSC curves show exothermic maxima near critical temperatures reported in the Mg-Cu phase diagram, along with decreases in exothermic peak temperatures and effective E_a values as bilayer spacing increases. Thermal trends indicate three distinct reaction regions that emerge as a function of bilayer thickness: (i) 1020 nm thick Mg-CuO samples react near the Mg melting point (∼650 °C) and have an effective E_a of ∼600 kJ/mol. (ii) Mg-CuO films with 510-255 nm thick bilayers exhibit reactions between 511 and 548 °C, with effective E_a values significantly decreasing to 128-162 kJ/mol. At these thicknesses, Mg-CuO reaction temperatures occur near the Mg₂Cu/Mg and Mg₂Cu/Cu₂Mg eutectic points of 483 and 551 °C, respectively, suggesting the presence of a liquid phase enhancing mass transport. (iii) Mg-CuO samples with 204-170 nm thick bilayers appear to react via a solid-state reaction process as they exhibit exothermic behavior between 388 and 421 °C and have the lowest effective E_a values of 102-106 kJ/mol. DSC results are corroborated by in situ HTXRD results on Mg-CuO films with bilayer thicknesses of 1 μm and 340 and 204 nm that show transient intermetallic peaks coinciding with Cu/Mg phase diagram predictions and the appearance of Cu peaks at 640, 530, and 415 °C, respectively. These findings indicate that factors such as reactant properties and bilayer thickness significantly influence observed reaction pathways and that material concepts such as phase diagrams may act as heuristic guides to developing new reactive materials.